This invention relates in general to current control circuits and in particular to current control circuits for controlling the sequential energization of an external load having at least two conductors.
A number of prior art methods and apparatus exist for driving stepping motors. One common problem associated with the driving of stepping motors is the problem of creating a constant current operation when the windings of the stepping motor are energized. One prior method used to obtain constant current operation through a selected energized winding is the use of a current sensing network in each winding set to turn off a high voltage switching transistor in each winding when the current reaches its desired value. The switching transistor turns on again to allow the current to flow through the selected stepper windings when the current decreases below the threshold. This method is undesirable because of the high cost of high-speed, high-current transistors.
The U.S. Patent to Kogler et al., U.S. Pat. No. 3,967,174 issued June 29, 1976, discloses a method and apparatus for controlling the current to a Y-connected stator winding. The Kogler patent, however, does not teach the use of a resettable timing circuit as part of its control circuit and therefore does not provide efficient cyclic control of the current to lower power losses and raise efficiency.
The subject invention seeks to hold the motor current at a predetermined level at each successive coil or winding in a stepper motor. Multiple power transistors and control circuits are avoided by the use of a control circuit based at a ground reference level. The control circuit also provides current control without the attendant cost or complexity of other current control circuits such as well known chopping circuits which use two power transistors per winding.
An electronic circuit is provided for controlling the energization of an external load having at least two conductors, a sequencing circuit selecting which one of the conductors is to be energized. An electronic circuit constructed in accordance with the instant invention includes a switching means for sequentially allowing the conductors to be energized whereby an electric current flows therethrough a selected one of the conductors. The electronic circuit also includes a control means responsive to a predetermined amount of current flowing through the selected conductor for interrupting the energization for a predetermined time period. The switching means is responsive to the control means to allow the energization to continue after the predetermined time period.